Scattering of ultrasound by biological tissues is not yet fully understood. Our long term objective is to gain such an understanding with the aim of increasing the utility of ultrasonic diagnostic procedures. In order to achieve this objective we will work on the following problem areas: We will continue to work on determining by analysis and computer simulations the extent of phase-cancellation artifacts in the measurement of scattered fields from different tissues. It is known that scattered fields, being Rayleigh distributed, are fundamentally characterized by large amplitude variations, since the standard deviation of a Rayleigh distributed random entity is 40% of its mean value. These large amplitude variations coupled with short coherence lengths and transducers that are many wavelengths in diameter lead to severe phase-cancellation artifacts in diagnostic imaging. The proposed research will also include calculation of the coherence lengths associated with the scattered ultrasound from different biological tissues. By using Monte Carlo techniques we will also perform computer simulations to determine the extent of multiple scattering in different tissues. Such techniques have already proved successful in handling the multiple scattering problems in the atmosphere. An important aim of the proposed research will be to conduct further studies on the new technique that we have developed for the imaging of scattering media. A distinguishing feature of our technique is that it does not require illumination of the object by plane, cylindrical or spherical waves which are difficult to generate in laboratory. In other words, our technique works with ordinary transducers. Also, in the transmission mode our technique requires the measurement of the scattered data for only two rotational positions of the object. Theoretically, the new technique should also work on reflected ultrasound. Computer simulation and experimental test of this claim will be conducted under the proposed program.